There has been disclosed a driving circuit for driving a DC brushless motor by employing a battery as its power source (see, e.g., Japanese Patent Application Publication No. H06-104000). In such a conventional example, when a control unit rotates a brushless motor by switching a bridge circuit including a plurality of field effect transistors (FETs) through a driver circuit based on a signal outputted from a position detection unit (not shown) of the brushless motor, a voltage to be supplied to gates of the FETs is generated by a floating voltage generator (bootstrap circuit).
In the meantime, as shown in FIG. 3, the higher voltage applied between the gate and the source (hereinafter, referred to as “gate/source voltage”) results in lowering the on-resistance and thus decreasing the heat dissipation. On the other hand, the lower gate/source voltage results in raising the on-resistance and thus increasing the heat dissipation. In the case of the FET shown in FIG. 3, it is preferable to apply the gate/source voltage of 7 V or higher.
On the other hand, the control unit is typically formed of a microcomputer, and its driving voltage is about 3 or 5 V, which is lower than the required gate/source voltage.
In the case of a battery-powered electric power tool, the battery voltage is high in a fully charged state and becomes lower as the charged amount of battery becomes reduced. To increase the amount of work per one charging cycle, it may be necessary to operate the electric power tool even when the charged amount of battery is low and, thus, the battery voltage is low. However, when the battery voltage is lowered, the gate/source voltage of FET also becomes lowered, which causes the increased heat dissipation and making unstable the operation of the FET.
Accordingly, when the battery voltage becomes lower than a reference voltage, it is necessary to stop the operation of the electric power tool. Since, however, the reference voltage is higher than an operable voltage range of the control unit, stopping the operation of the electric power tool with reference to the reference voltage brings about the decrease in the amount of work per single charging cycle.